nanomaterials Article High Figure of Merit Optical Buffering in Coupled-Slot Slab Photonic Crystal Waveguide with Ionic Liquid Israa Abood 1,2, Sayed Elshahat 2,3,4 and Zhengbiao Ouyang 1,2,* 1 College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China; [email protected] 2 THz Technical Research Center of Shenzhen University, Shenzhen. Key Laboratory of Micro-nano Photonic Information Technology, Key Laboratory of Optoelectronics Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen 518060, China; [email protected] 3 Institute of Microscale Optoelectronics, Shenzhen University, Shenzhen 518060, China 4 Physics Department, Faculty of Science, Assiut University, Assiut 71516, Egypt * Correspondence: [email protected] Received: 8 July 2020; Accepted: 26 August 2020; Published: 3 September 2020 Abstract: Slow light with adequate low group velocity and wide bandwidth with a flat band of the zero-dispersion area were investigated. High buffering capabilities were obtained in a silicon-polymer coupled-slot slab photonic crystal waveguide (SP-CS-SPCW) with infiltrating slots by ionic liquid. A figure of merit (FoM) around 0.663 with the lowest physical bit length Lbit of 4.6748 µm for each stored bit in the optical communication waveband was gained by appropriately modifying the square air slot length. Posteriorly, by filling the slots with ionic liquid, the Lbit was enhanced to be 4.2817 µm with the highest FoM of 0.72402 in wider transmission bandwidth and ultra-high bit rate in terabit range, which may become useful for the future 6G mobile communication network. Ionic liquids have had a noticeable effect in altering the optical properties of photonic crystals. A polymer was used for the future incorporation of an electro-optic effect in buffers to realize the dynamic controlling of optical properties. Ionic liquids enhanced the transmission rate through optical materials. Additionally, the delay time in the ns-range was achieved, providing longer delay and ultra-low group velocity, which is important for light-matter interaction in light amplifiers and nonlinear devices. Keywords: slow light; buffering capability; photonic crystal waveguide; ionic liquids 1. Introduction Incoming polymer technology has provided attractive candidates for photonic materials due to their low-cost production and wide bandwidth [1–3]. Wide bandwidth, dispersion-free propagation in a photonic crystal waveguide (PCW) is a worthy nominee for slow-light implementation, which reinforces the trapping and guiding of optical pulses. Slow light encourages stronger light-matter interaction while providing additional control over the spectral bandwidth for this interaction and allowing delaying and temporarily storing of light [4]. Room temperature ionic liquids (RTILs) are carbon-based salts that are in a fluid state at room temperature. As a new polar solvent, ionic liquid has almost no vapor pressure, is nonvolatile and noninflammable, and has a low melting point below 100 ◦C[5]. RTILs possess strong electrostatic field, good electrical and thermal conductivity, good light transmittance and high refractive index [6,7], good chemical stability and thermal stability, and are recyclable and environmentally friendly. So, it is called “green” chemical solvent to replace the traditional volatile toxic solvent [8,9]. Therefore, ionic liquids are considered as one of the most promising green solvents and catalysts in the 21st century and have been applied in many fields such as biocatalysis, separation technology, and electrochemistry [10]. Nanomaterials 2020, 10, 1742; doi:10.3390/nano10091742 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1742 2 of 12 Furthermore, some polymerized RTILs can work as the final material for passive optical components. Such material can combine the benefits of polymers with that of dyes and nanoparticles. Polymerized RTILs are incompletely transparent in the visible waveband and completely transparent in the near-IR. Several RTILs with relatively low indices (n f < 1.50) and also high-index RTILs ((n f > 1.70) are commercially available [5]. These ILs are used as immersion liquids to tune the optical properties of photonic crystals (PhCs) or to match the refractive indices of the constituting materials of these photonic crystals. Moreover, the refractive indices of polymerized RTILs have been determined because they are important for producing microstructures in photonics. Their refractive index is about 1.6–1.7 in the wavelength range of 600–2000 nm for both polymerized RTILs, which include optical communication wavelength 1550 nm evaluated based on ellipsometry measurements [11]. From the optical properties of polymerized RTILs, they seem to be promising materials for the fabrication of infrared photonic elements. The RTIL-based planar optical components can be realized on glass substrates because of the relatively high refractive indices of RTIL-derived polymers (about 1.65) [5]. The potential applications for micro-optical components include waveguides, micro-resonators, and photonic crystals, which are essential components in photonic integrated circuits (PICs) [12]. Different microfabrication techniques have been applied in the technology of polymer-based micro-optical devices, including electron beam lithography (EBL) and soft lithography [13] or electron beam etching [14]. Lately, RTIL is used instead of polymers as an electron beam (EB)-sensitive material [12,15]. The conditions of the fabrication would be chosen carefully to minimize the fabrication errors of the designed parameters. Therefore, it is necessary to use some processes with high precision, such as high-resolution lithography and high-quality pattern. Thus, the direct writing method by electrons is attractive, particularly in terms of obtaining high resolution without using additional etching steps or photolithography masks [12]. Consequently, we considered polymerized RTILs in our calculations based on silicon-polymer, coupled-slot, slab photonic crystal waveguide (SP-CS-SPCW), which exploits the benefits of both coupled-slot cavity and RTILs infiltrated. 2. Theoretical Characteristics The performance and properties of slow light are often specified with more significant parameters. The first is the group velocity of guided mode, which is obtained by the well-known dispersion relationship: d! c vg = = , (1) dk ng where ! and k are the angle frequency and wavevector of the incident light, c is the speed of light in free space, and ng is the group index inside the PhC medium. The second is the group-velocity dispersion (GVD, or β2): d2k a dng GVD = β2 = = . (2) d!2 2πc2 dU where U = !a/2πc is the normalized frequency and a is the lattice constant. The third is the normalized delay bandwidth product (NDBP), which is the most important parameter for slow-light performance. It can be calculated as the multiplication of the average group index ng per frequency range, where ng is restricted by over 10% variation of group index [16,17] and the bandwidth ratio (DU/U ) [18]: ± 0 D! DU NDBP = ng = ng . (3) × !0 × U0 where D! and !0 are the bandwidth and central frequency, respectively; DU = D!a/2πc and U0 = !0a/2πc are the normalized bandwidth and normalized central frequency, respectively. NDBP is an indispensable quantity in representing the slow-light performance and more often used when the devices have different lengths and/or operating frequencies [19–22]. Buffering performance includes several parameters related to dispersion curves: Storage time Ts in the waveguide, physical bit length Lbit, buffer capacity C, and bit rate Rb. The optical buffer is Nanomaterials 2020, 10, 1742 3 of 12 used to store data and adjust the timing of optical packets. For the PhC slab waveguide length of L, the storage time can be obtained by Ts = L/vg. The bit rate can be calculated by [23] c DU R = B /2 = × , (4) b packet 4a where Bpacket is the baseband bandwidth of the input data. It is important to find the balance between device bit length and bit rate. The average buffer capacity (C) can be calculated by L ng c DU LU0 C = Ts B = × × = NDBP. (5) packet c × 2a 2a × For all delay-line buffers, the minimum physical size of the buffer for a given number of buffered data bits is ultimately limited by the physical size of each stored bit. All slow-light optical buffers require a lower value of the stored bit length. The materialistic size of a bit stored, the bit length, can be calculated as [24]: L 2a L = = . (6) bit C U NDBP 0 × From Equations (5) and (6), the remarkable figure of merit (FoM) for all slow-light devices and an optical buffer is NDBP, which requires a lower value of the stored bit length. 3. Structure Geometry and Simulation Results The proposed SP-CS-SPCW structure is based on a triangular lattice photonic crystal made from a silicon substrate with refractive index nsi = 3.48 and basic holes filled with highly nonlinear poled electro-optic polystyrene of npoly = 1.59, with elementary radius R = 0.30a. The coupled-slot cavity waveguide S1 is first constituted by inserting square air slots of length l into the center of the waveguide, as shown in Figure1a. Afterward, the square air slots will be filled by the ionic liquid. The spacing between each two neighboring square slots is 3a. Suppose, for example, that the dielectric constant is independent on the Y-axis. At that point, the solutions of all take the form of either transverse magnetic (TM) modes with nonzero Hy, Ex, Ez (H field refer to Magnetic component and E field refer to Electric component.) or transverse electric (TE) modes with nonzero Hx, Hz, Ey . Our proposed structures support TM only. Figure1b displays the dispersion curve for TM mode of the square slot defect at l = 1.30a of S1, i.e., the first derivative between the normalized frequency U = !a/2πc = a/λ and the normalized wavevector K = ka/2π for the dispersion relation of the guided mode.